Magnesium alloy sheet material

ABSTRACT

Disclosed is a magnesium alloy material having excellent tensile strength and favorable ductility. Therefore, the magnesium alloy sheet material formed by rolling a magnesium alloy having a long period stacking order phase crystallized at the time of casting includes in a case where a sheet-thickness traverse section of an alloy structure is observed at a substantially right angle to the longitudinal direction by a scanning electron microscope, a structure mainly composed of the long period stacking order phase, in which at least two or more αMg phases having thickness in the observed section of 0.5 μm or less are laminated in a layered manner with the sheet-shape long period stacking order phase.

TECHNICAL FIELD

The present invention relates to a magnesium alloy sheet material. Indetail, the present invention relates to a magnesium alloy sheetmaterial having high tensile strength and high ductility.

BACKGROUND ART

In general, a magnesium alloy has the lowest density and the lightestweight and also has high tensile strength among practically utilizedalloys. Thus, magnesium alloy is increasingly applied to a casing of anelectric product, a wheel, a suspension, and parts around an engine ofan automobile, and the like.

Particularly, high mechanical properties are required for parts used inrelation to automobiles. Thus, as a magnesium alloy to which elementssuch as Gd and Zn are added, a material of a specific form ismanufactured by a single roll method and a rapid solidification method(for example, refer to Patent Document 1 and Patent Document 2).

However, regarding the magnesium alloy described above, although highmechanical properties are obtained with a specific manufacturing methodthere is a problem that special facilities are required in order torealize the specific manufacturing method and moreover, productivity islow. Furthermore, there is a problem that applicable members arelimited.

Conventionally, there is a proposed technique that in a case ofmanufacturing a magnesium alloy, even when highly-productive normalmelting and casting and then plastic working (extrusion) are performedwithout using the special facilities or processes as described in PatentDocument 1 and Patent Document 2 above, practically useful mechanicalproperties are obtained (for example, refer to Patent Document 3).

CITATION LIST

Patent Document

-   Patent Document 1: Japanese Published Unexamined Patent Application    No. H6-41701-   Patent Document 2: Japanese Published Unexamined Patent Application    No. 2002-256370-   Patent Document 3: Japanese Published Unexamined Patent Application    No. 2006-97037

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

A magnesium alloy having a long period stacking order phase(hereinafter, referred to as the “LPSO” phase) disclosed in PatentDocument 3 is excellent in balance between tensile strength andductility. Although a cast material does not have very high tensilestrength, by performing plastic working such as extrusion, improvementin tensile strength can be realized without lowering ductility verymuch. That is, even when plastic working of a large working ratio suchas extrusion is performed, sufficient ductility can be obtained.

However, when tensile strength is to be improved with plastic working atthe time of manufacturing a sheet material or a rod material as amaterial, ductility is consequently lowered.

For example, FIG. 6 shows yield strength, tensile strength, andelongation of a cast material of a Mg₉₆ZnY₃ alloy and hot-rolledmaterials (R1, P2). It is found that the hot-rolled material (P2) hashigher yield strength and higher tensile strength but smaller elongationthan the hot-rolled material (R1). It should be noted that FIG. 6 isdescribed in Non-patent Document (R. G. Li, D. Q. Fang, J. An, Y. Lu, Z.Y. Cao, Y. B. Liu, MATERIALS CHARACTERIZATION 60 (2009) 470-475).

FIG. 7 shows mechanical properties of various materials. When themechanical properties of the same alloys of different processes arecompared, it is found that alloys realizing high yield strength and hightensile strength have small elongation. It should be noted that FIG. 7is described in Non-patent Document (T. Itoi et al./Scripta Materialia59 (2008) 1155-1158).

As described above, both the characteristics of tensile strength andductility are not easily improved at the same time.

The present invention has been made in view of these circumstances, andan object thereof is to provide a magnesium alloy sheet material capableof realizing improvement in tensile strength and at the same time, alsorealizing improvement in ductility.

Means for Solving the Problem

In order to achieve the above object, a magnesium alloy sheet materialof the present invention is a magnesium alloy sheet material formed byrolling a magnesium alloy having a long period stacking order phasecrystallized at the time of casting, including, in a case where asheet-thickness traverse section of an alloy structure is observed at asubstantially right angle to the longitudinal direction by a scanningelectron microscope, a structure mainly composed of the long periodstacking order phase, in which at least two or more αMg phases havingthickness in the observed section of 0.5 μm or less are laminated in alayered manner with the sheet-shape long period stacking order phase.

Here, in a case where the sheet-thickness traverse section of the alloystructure is observed at a substantially right angle to the longitudinaldirection by the scanning electron microscope, the structure mainlycomposed of the long period stacking order phase, in which at least twoor more αMg phases having thickness in the observed section of 005 μm orless are laminated in a layered mariner with the sheet-shape long periodstacking order phase is provided, improvement in tensile strength can berealized and at the same time, improvement in ductility can also berealized, so that excellent tensile strength and favorable ductility canbe realized.

That is, the LPSO phase is formed in a sheet shape (plate shape). Thus,when comparing with a case where the LPSO phase is formed in a blockshape, at least part of the LPSO phase is brought into a structure statethat the part is easily shear-deformed or compression-deformed inaccordance with rolling. In addition, since at least part of the LPSOphase is in the structure state that the part is easily shear-deformedor compression-deformed, a kink band is easily introduced into the LPSOphase, and as a result, excellent tensile strength can be realized. Inaddition, since at least part of the LPSO phase is in the structurestate that the part is easily shear-deformed or compression-deformed,favorable ductility can also be realized.

In a case where maximum sheet thickness of the LPSO phase in thelaminated structure is 9 μm or less, generally 10% or more elongationcan be realized.

Furthermore, in a case where the laminated structure (specifically, theLPSO phase or the αMg phases) includes an intermetallic compound (suchas Mg₃Zn₃Y₂), the structure state is such that the intermetalliccompound is sandwiched by the sheet-shape (plate-shape) LPSO phase.Since the intermetallic compound easily facilitates deformation of theLPSO phase, such a structure state is a state that the LPSO phase iseasily deformed. Therefore, the kink band is easily introduced into theLPSO phase, so that excellent tensile strength can be realized.

When at least part of the laminated structure is shear-deformed orcompression-deformed, at least part of the laminated structure is curvedor bent. Such a curved or bent structure can be a cause for realizingexcellent tensile strength.

Here, the “sheet-shape LPSO phase in a case where the sheet-thicknesstraverse section of the alloy structure is observed at a substantiallyright angle to the longitudinal direction by the scanning electronmicroscope” indicates a structure as shown in FIG. 8, for example. Alight gray point in FIG. 8 indicates the LPSO phase. It should be notedthat FIG. 8( a) is a scanning electron micrograph of a magnification of150×, FIG. 8( b) is a scanning electron micrograph of a magnification of2,500×, and FIG. 8( c) is a scanning electron micrograph of amagnification of 3,000×.

The “sheet-thickness traverse section” indicates a section whosethickness is reduced by rolling, the section which is substantiallyparallel to the forward direction of the sheet material at the time ofrolling (section at a substantially right angle to a mill roll).Furthermore, the “longitudinal direction of the sheet-thickness traversesection” indicates the direction which is substantially parallel to theforward direction of the sheet material at the time of rolling(direction at a substantially right angle to the rolling roll). The“substantially right angle to the longitudinal direction of thesheet-thickness traverse section” indicates the thickness direction ofthe sheet-thickness traverse section.

That is, the “sheet-thickness traverse section is observed at asubstantially right angle to the longitudinal direction” indicates thatthe “‘section whose thickness is reduced by rolling, the section whichis substantially parallel to the forward direction of the sheet materialat the time of rolling’ is observed in the ‘thickness direction of thesection’ at the substantially right angle to the ‘direction which issubstantially parallel to the forward direction of the sheet material atthe time of rolling.’”

The “magnesium alloy in which the LPSO phase is crystallized at the timeof casting” includes Mg—Zn-RE (RE=Y, Dy, Ho, Er, Tm), Mg—Cu-RE (RE=Y,Gd, Tb, Dy, Ho, Er, Tm), Mg—Ni-RE (RE=Y, Sm, Gd, Tb, Dy, Ho, Er),Mg—Co-RE (RE=Y, Dy, Ho, Er, Tm), and Mg—Al—Gd. It should be noted thatRE indicates a rare-earth element.

Furthermore, the “magnesium alloy in which the LPSO phase iscrystallized at the time of casting” is not necessarily limited to athree-component system as exemplified above but may be a four-componentsystem in which another additive element is added to the magnesium alloydescribed above or a larger component system.

Effects of the Invention

With the magnesium alloy sheet material of the present invention, theimprovement in tensile strength can be realized and at the same time,the improvement in ductility can also be realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A (a) is a micrograph (1) showing a crystalline structure of aMg₉₆Zn₂Y₂ alloy serving as a magnesium alloy sheet material of thepresent invention;

FIG. 1A(b) is a micrograph (2) showing the crystalline structure of theMg₉₆Zn₂Y₂ alloy serving as the magnesium alloy sheet material of thepresent invention;

FIG. 1A(c) is a micrograph (3) showing the crystalline structure of theMg₉₆Zn₂Y₂ alloy serving as the magnesium alloy sheet material of thepresent invention;

FIG. 1B(a) is a micrograph (4) showing the crystalline structure of theMg₉₆Zn₂Y₂ alloy serving as the magnesium alloy sheet material of thepresent invention;

FIG. 1B(b) is a micrograph (5) showing the crystalline structure of theMg₉₆Zn₂Y₂ alloy serving as the magnesium alloy sheet material of thepresent invention;

FIG. 1B(c) is a micrograph (6) showing the crystalline structure of theMg₉₆Zn₂Y₂ alloy serving as the magnesium alloy sheet material of thepresent invention;

FIG. 2 is a flowchart for illustrating a manufacturing method of themagnesium alloy sheet material;

FIG. 3 is a micrograph for illustrating an intermetallic compoundMg₃Zn₃Y₂;

FIG. 4A is a micrograph (1) showing a crystalline structure of themagnesium alloy material formed by performing rolling S4 on aplastically-worked item to which no heating step is performed;

FIG. 4B(a) is a micrograph (2) showing the crystalline structure of themagnesium alloy material formed by performing the rolling S4 on theplastically-worked item to which no heating step is performed;

FIG. 41B(b) is a micrograph (3) showing the crystalline structure of themagnesium alloy material formed by performing the rolling S4 on theplastically-worked item to which no heating step is performed;

FIG. 4B (c) is a micrograph (4) showing the crystalline structure of themagnesium alloy material formed by performing the rolling S4 on theplastically-worked item to which no heating step is performed;

FIG. 5 is a graph showing 0.2% yield strength, tensile strength, andelongation of Example and Comparative Example;

FIG. 6 is a graph showing yield strength, tensile strength, andelongation of a cast material of a Mg₉₆ZnY₃ alloy and hot-rolledmaterials (R1, R2);

FIG. 7 is a table showing mechanical properties of various materials;

FIG. 8( a) is a micrograph (1) for illustrating one example of asheet-shape structure;

FIG. 8( b) is a micrograph (2) for illustrating one example of thesheet-shape structure;

FIG. 8( c) is a micrograph (3) for illustrating one example of thesheet-shape structure;

FIG. 9 is a graph showing a relationship between a heating time andtensile yield strength and a relationship between the heating time androom temperature elongation;

FIG. 10( a) is a diagram (1) for illustrating a relationship betweenmaximum thickness of an LPSO phase in a lamellar structure andelongation of the magnesium alloy sheet material;

FIG. 10( b) is a diagram (2) for illustrating the relationship betweenthe maximum thickness of the LPSO phase in the lamellar structure andelongation of the magnesium alloy sheet material;

FIG. 11A(a) is a scanning electron micrograph (1) of the magnesium alloysheet material formed by rolling an excessively heated material;

FIG. 11A(b) is a scanning electron micrograph (2) of the magnesium alloysheet material formed by rolling the excessively heated material;

FIG. 11B (a) is a scanning electron micrograph (3) of the magnesiumalloy sheet material formed by rolling the excessively heated material;and

FIG. 11B (b) is a scanning electron micrograph (4) of the magnesiumalloy sheet material formed by rolling the excessively heated material.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an embodiment of the present invention will be describedwith reference to the drawings for understanding of the presentinvention.

FIGS. 1A and 1B are scanning electron micrographs showing a crystallinestructure of a Mg₉₆Zn₂Y₂ alloy serving as a magnesium alloy sheetmaterial of the present invention. In FIGS. 1A and 1B, a αMg phase isblack, an LPSO phase is gray, and a Mg₃Zn₃Y₂ is white.

It should be noted that in the present embodiment, description will begiven taking the Mg₉₆Zn₂Y₂ alloy as an example. However, the presentinvention is not limited to such an alloy composition. For example,another three-component system or a four-component system in whichanother additive element is added may be adopted.

As clear from FIGS. 1A and 1B, the magnesium alloy sheet material towhich the present invention is applied has an LPSO phase and αMg phases,and the LPSO phase and the αMg phases are formed in a lamellar manner.However, not all the structures are lamellar structures but a regionshown by reference sign X in FIG. 1A(c) is not the lamellar structure.

It should be noted that the LPSO phase is a precipitate precipitated ina grain and a grain boundary of a magnesium alloy, which is a structuralphase that sequence of bottom surface atomic layers in an HCP structureis repeated in the bottom surface normal direction with a long periodorder, that is, a long period stacking order phase. By precipitation ofthis LPSO phase, mechanical properties of the magnesium alloy sheetmaterial (tensile strength, 0.2% yield strength, and elongation) areimproved.

The LPSO phase has a sheet-shape (plate-shape) structure (regions shownby reference sign S in FIG. 15( b)). The αMg phase is placed in a gapbetween the sheet-shape (plate-shape) structure. That is, thesheet-shape (plate-shape) structure is laminated as multiple layers inthe LPSO phase.

Specifically, the lamellar structure described above in the magnesiumalloy sheet material to which the present invention is applied (refer toreference sign S in FIG. 1B (b) is mainly composed of the LPSO phase,and in a case where a sheet-thickness traverse section is observed at asubstantially right angle to the longitudinal direction by a scanningelectron microscope, the plurality of αMg phases having thickness in theobserved section of 0.5 μm or less and the sheet-shape (plate-shape)LPSO phase are laminated in a layered manner. It should be noted that ina case where the sheet-thickness traverse section observed at asubstantially right angle to the longitudinal direction by the scanningelectron microscope, the sheet-shape (plate-shape) LPSO phase hasthickness of 0.25 μm or more in the observed section.

Regarding the lamellar structure described above (refer to referencesign S in FIG. 1B(b)), by appropriately heating a material thereof (suchas an extrusion material) before rolling, the structure of the LPSOphase can be controlled to have a desired sheet shape (plate shape).

FIG. 9( a) shows a “relationship between a heating time and tensileyield strength,” and FIG. 9( b) shows a “relationship between theheating time and room temperature elongation.” It should be noted that aheating temperature is 480° C. As clear from the “relationship betweenthe heating time and the room temperature elongation” shown in FIG. 9(b), the elongation is not improved by simply heating but there is a needfor appropriately heating in such a manner that a thin sheet materialafter rolling can realize large elongation.

FIG. 10( a) shows a “relationship between maximum thickness of the LPSOphase in the lamellar structure and elongation of the magnesium alloysheet material.” As clear from FIG. 10( a), in a case where thestructure is refined so that the maximum thickness in the observedsection of the LPSO phase in the lamellar structure is 9 μm or less,generally 10% or more elongation can be obtained.

That is, by appropriately heating before rolling, it is extremelyimportant technically that the maximum thickness in the observed sectionof the LPSO phase in the lamellar structure after rolling is 9 μm orless.

It should be noted that the “thickness in the observed section of theLPSO phase” indicates length in the perpendicular direction to thelongitudinal direction of the sheet-shape (plate-shape) LPSO phase(direction of arrow shown in FIG. 10( b)).

A heating condition before rolling is appropriately selected. Then, evenwith the structure in which the thickness in the observed section of theLPSO phase in the lamellar structure looks large, in a case whereconfirmation is performed with a magnification of the scanning electronmicroscope being increased, the αMg phases of thin films of 0.1 μm orless than 0.1 μm form a laminated structure together with the LPSOphase. That is, a multilayer structure in which the LPSO phase of a thinfilm and the αMg phases having smaller thickness in the observed sectionthan the LPSO phase are laminated can be confirmed.

Meanwhile, by insufficient heating, the sheet-shape (plate-shape) LPSOphase cannot sufficiently be formed. By excessive heating such as a longheating time, the thickness in the observed section of the sheet-shape(plate-shape) LPSO phase is increased, so that a formation frequency ofthe layer structure with the thin αMg phases is lowered (refer to FIGS.11A and 11B).

FIGS. 11A and 11B show scanning electron micrographs of the magnesiumalloy sheet material formed by rolling an excessively heated material.It should be noted that in order to improve convenience in visualrecognition, FIGS. 11A(a) and 11B(a) show states in which a contrast ofthe LPSO phase is enhanced and FIGS. 11A(b) and 11B(b) show states inwhich a contrast of the compound is enhanced.

In the magnesium alloy sheet material to which the present invention isapplied, by appropriately heating the material thereof before rolling asin a manufacturing method described below, the structure is controlledso that the thickness in the observed section of the LPSO phase in thelamellar structure, in other words, the thickness in the observedsection of the LPSO phase not sandwiching the αMg phase of a thin filmof 0.5 μm or less is 8 μm at maximum.

The LPSO phase has the sheet shape (plate-shape) structure. Thus, whencomparing with an LPSO phase having a block shape structure, at leastpart of the LPSO phase is easily shear-deformed or compression-deformedin accordance with rolling. It should be noted that the fact that atleast part of the LPSO phase is easily shear-deformed orcompression-deformed in accordance with rolling is clear from the factthat part of the lamellar structure of the LPSO phase and αMg phases iscurved or bent as described below.

Since at least part of the LPSO phase is in a structure state that thepart is easily shear-deformed or compression-deformed in accordance withrolling, a kink band is easily introduced into the LPSO phase as aresult, so that excellent tensile strength can be realized. Since atleast part of the LPSO phase is in the structure state that the part iseasily shear-deformed or compression-deformed, in accordance withrolling, favorable ductility can also be realized.

It should be noted that the LPSO phase not only has the sheet-shape(plate-shape) structure but also sometimes has a block-shape structureas in a region shown by reference sign Y in FIG. 1A(b), for example.That is, a structure shape of the LPSO phase is a sheet shape(plate-shape) or a mixture of a sheet shape (plate-shape) and a blockshape.

It is found that in both the LPSO phase and the αMg phases of thelamellar structure, the structure is totally curved. This is thought tobe because the structure or part of the structure is curved or bent dueto shear-deformation or compression-deformation of the sheet-shape(plate-shape) LPSO phase and the αMg phases sandwiched by such asheet-shape (plate-shape) LPSO phase (region shown by reference sign Tin FIG. 1B(b)). It should be noted that curving or bending of thelamellar structure can be a cause for realizing excellent tensilestrength.

Furthermore, Mg₃Zn₃Y₂ is minutely spread in the LPSO phase or the αMgphases (regions shown by reference sign Z in FIGS. 1A(b) and 1A(c) andregions shown by reference sign T and reference sign U in FIG. 1B(c)).

The intermetallic compound Mg₃Zn₃Y₂ is in a structure state that thecompound is sandwiched by the LPSO phase. The LPSO phase has thesheet-shape (plate-shape) structure. Therefore, the intermetalliccompound Mg₃Zn₃Y₂ facilitates deformation of the LPSO phase. Thus, as aresult of facilitation of the deformation of the LPSO phase, the kinkband is easily introduced into the LPSO phase, so that excellent tensilestrength can be realized.

As described above, in the magnesium alloy sheet material of the presentinvention, the IPSO phase has the sheet-shape (plate-shape) structureand is in the structure state that the LPSO phase is easilyshear-deformed or compression-deformed in accordance with rolling, andthe intermetallic compound Mg₃Zn₃Y₂ facilitates the deformation of theLPSO phase. Thus, improvement in tensile strength can be realized and atthe same time, improvement in ductility can also be realized.

In the magnesium alloy sheet material of the present invention, the LPSOphase is minutely spread by appropriate heating in order to obtain largeelongation, and without destroying the LPSO phase by strongshear-deformation or compression-deformation by rolling serving as thefollowing step, distortion, that is, kink deformation is effectivelygiven to the LPSO phase. Thus, a reinforcing mechanism of the LPSO phasecan sufficiently be activated. Therefore, the magnesium alloy sheetmaterial with the same working ratio of rolling but having largerelongation can be obtained.

Hereinafter, the manufacturing method of the magnesium alloy sheetmaterial of the present invention will be described.

FIG. 2 is a flowchart for illustrating the manufacturing method of themagnesium alloy sheet material of the present invention. As shown inFIG. 2, in the manufacturing method of the magnesium alloy sheetmaterial, of the present invention, casting is first performed in acasting step S1. In the casting step S1, a Mg—Zn—Y alloy containing Znand Y, and the remaining part including Mg and unavoidable impurities iscast, so as to form a cast material containing the LPSO phase and theαMg phases.

It should be noted that a forming method of the cast material may be anymethod such as a method of high-frequency induction melting in an Ar gasatmosphere (refer to Example 1 of International Publication No,2007/111342) and a method for melting a magnesium alloy while making aCO₂ gas flow into an iron crucible using an electric furnace, andcharging the alloy into an iron casting mold (refer to Example 3 ofInternational Publication No, 2007/111342).

It is found that in a case where the Mg % Zn₂Y₂ alloy is cast, theintermetallic compound Mg₃Zn₃Y₂ of approximately 0.5 μm to 2.0 μm isformed at a time of casting. It should be noted that FIG. 3( a) is ascanning electron micrograph showing a crystalline structure of anannealed material of the Mg₉₆Zn₂Y₂ alloy at 400° C. for one hour, FIG.3( b) is a scanning electron micrograph showing a crystalline structureof the annealed material of the Mg₉Zn₂Y₂ alloy at 450° C. for one hour,FIG. 3( c) is a scanning electron micrograph showing a crystallinestructure of the annealed material of the Mg₉₆Zn₂Y₂ alloy at 500° C. forone hour, and it is found that the intermetallic compound Mg₂Zn₃Y₂ isformed. It should be noted that the points indicated by reference signsa in the micrographs shown in FIGS. 3( a) to 3(c) indicate intermetalliccompounds Mg₃Zn₃Y₂.

Next, a plastic working step S2 is performed on the cast material.Plastic working of this plastic working step S2 is, for example,extrusion, casting, rolling, drawing, or the like. In aplastically-worked item obtained by performing plastic working on thecast material containing the LPSO phase, tensile strength, 0.2% yieldstrength, and elongation are improved in comparison to before plasticworking.

Successively, by performing a heating step S3 of heating theplastically-worked item, the LPSO phase is formed in a sheet shape(plate shape). As one example, heating is performed within a temperaturerange of 400° C. or more and 500° C. or leis and within a time range of0.5 hours or more and 10 hours or less, for example.

It should be noted that the LPSO phase is formed in a sheet shape (plateshape) by the heating step S3. However, it is only necessary to form theLPSO phase in a sheet shape (plate shape) prior to a rolling step S4described below in order to realize the crystalline structure shown inFIGS. 1A and 1B. Therefore, as long as the LPSO phase can be formed in asheet shape (plate shape), the heating step S3 is not always requiredbut any method may be used. Similarly, since it is only necessary toform the LPSO phase in a sheet shape (plate shape), the presentinvention is not limited to the temperature range and the time rangeexemplified above.

Thereafter, by performing the rolling S4 on the plastically-worked itemheated so as to form the LPSO phase in a sheet shape (plate shape), themagnesium alloy sheet material of the present invention as shown inFIGS. 1A and 1B can be obtained.

FIGS. 4A and 4B are micrographs showing a crystalline structure of themagnesium alloy sheet material formed by performing the rolling S4 onthe plastically-worked item to which no heating step is performed. InFIGS. 4A and 4B, the αMg phase is black, the LPSO phase is gray, andMg₃Zn₃Y₂ is white.

As clear from FIGS. 4A and 4B, regarding the magnesium alloy sheetmaterial formed by performing the rolling S4 on the plastically-workeditem to which no heating step S3 is performed and in which the LPSOphase is not formed in a sheet shape (plate shape), the LPSO phase andthe αMg phases are formed in a lamellar manner.

However, as clear from FIGS. 4A(b) and 4A(c), regarding the sheet-shapestructure of the magnesium alloy material formed by performing therolling S4 on the plastically-worked item to which no heating step S3 isperformed and in which the LPSO phase is not formed in a sheet shape(plate shape), the LPSO phase is formed in a block shape, and the LPSOphase minutely spread in the αMg phases is extremely small. As clearfrom FIGS. 4E (b) and 4E (c), the LPSO phase is straight and no curvedor bent part is found.

It should be noted that the manufacturing method of the magnesium alloysheet material described above is only one example, and the magnesiumalloy sheet material may be manufactured by various other manufacturingmethods as a matter of course. The magnesium alloy of the presentinvention is not limited to the alloy obtained by the manufacturingmethod described above.

EXAMPLE

Hereinafter, an example and a comparative example of the presentinvention will be described. It should be noted that the example shownbelow is only one example and does not limit the present invention.

Example

First, as a magnesium alloy sheet material of the example of the presentinvention, a Mg—Zn—Y alloy containing 2 atom % of Zn, 2 atom % of Y, andthe remaining part including Mg and unavoidable impurities was melted ina high-frequency melting furnace. Next, the heated and melted materialwas cast by a mold, so that an ingot (cast material) of φ69 mm×L200 mmwas produced. Furthermore, plastic working (extrusion) was performed atan extrusion temperature of 350° C. at an extrusion ratio of 10, so thatthe ingot was made into a sheet form. Successively, one-hour heating(annealing) was performed at a heating temperature of 100° C. to 500°C., so that an LPSO phase was formed in a sheet shape (plate shape).Thereafter, rolling was performed, so that a test piece was produced.

A result of a tensile test performed on the magnesium alloy sheetmaterial obtained in such a way at a room temperature and an evaluationof mechanical properties is shown in FIG. 5(b). It should be noted thatreference sign A in FIG. 5 indicates 0.2% yield strength, reference signB in FIG. 5 indicates tensile strength, and reference sign C in FIG. 5indicates ductility.

Comparative Example

Next, as a magnesium alloy sheet material of the comparative example, aMg—Zn—Y alloy containing 2 atom % of Zn, 2 atom % of 1, and theremaining part including Mg and unavoidable impurities was melted in ahigh-frequency melting furnace. Next, the heated and melted material wascast by a mold, so that an ingot (cast material) of (φ69 mm×L200 mm wasproduced. Furthermore, plastic working (extrusion) was performed at anextrusion temperature of 350° C. at an extrusion ratio of 10, so thatthe ingot was made into a sheet form. Thereafter, without forming anLPSO phase in a sheet shape (plate shape), rolling was performed, sothat a test piece was produced.

A result of a tensile test performed on the magnesium alloy sheetmaterial obtained in such a way at the room temperature and anevaluation of mechanical properties is shown in FIG. 5( a). It should benoted that reference sign A in FIG. 5 indicates 0.2% yield strength,reference sign B in FIG. 5 indicates tensile strength, and referencesign C in FIG. 5 indicates ductility.

As clear from FIG. 5, it is found that in the magnesium alloy sheetmaterial of the example of the present invention, both 0.2% yieldstrength and tensile strength are improved in comparison to themagnesium alloy sheet material of the comparative example. It is foundthat ductility is also improved. That is, with the magnesium alloy sheetmaterial of the example of the present invention, tensile strength andductility are improved at the same time without changing an alloycomposition in the magnesium alloy sheet material containing the LPSOphase.

1. A magnesium alloy sheet material formed by rolling a magnesium alloyhaving a long period stacking order phase crystallized at the time ofcasting, comprising: in a case where a sheet-thickness traverse sectionof an alloy structure is observed at a substantially right angle to thelongitudinal direction by a scanning electro microscope, a structuremainly composed of the long period stacking order phase, in which atleast two or more αMg phases having thickness in the observed section of0.5 μm or less are laminated in a layered manner with the sheet-shapelong period stacking order phase. 2.-7. (canceled)
 8. The magnesiumalloy sheet material according to claim 1, comprising 2 atom % of Zn, 2atom % of Y, and the remaining part including Mg and unavoidableimpurities.
 9. The magnesium alloy sheet material according to claim 1,wherein regarding the structure mainly composed of the long periodstacking order phase, in a case where a section cut in the sheetthickness direction along the rolling direction of the magnesium alloysheet material is observed from the perpendicular direction of thesection, a range of an area ratio of the long period stacking orderphase is 36% or more.
 10. The magnesium alloy sheet material accordingto claim 1, wherein the long period stacking order phase in thelaminated structure has maximum thickness in the observed section of 9μm or less.
 11. The magnesium alloy sheet material according to claim 1,wherein in the laminated structure, the sheet-shape long period stackingorder phase and the αMg phases having smaller-thickness in the observedsection than the long period stacking order phase are laminated in alayered manner.
 12. The magnesium alloy sheet material according toclaim 1, wherein the sheet-shape long period stacking order phase in thelaminated structure has minimum thickness in the observed section of0.25 μm or more.
 13. The magnesium alloy sheet material according toclaim 1, wherein the laminated structure includes an intermetalliccompound.
 14. The magnesium alloy sheet material according to claim 1,wherein at least part of the laminated structure is shear-deformed orcompression-deformed.
 15. The magnesium alloy sheet material accordingto claim 1, wherein at least part of the laminated structure is curvedor bent.